Silverstein thanked nearly everyone connected to the massive project, from Gov. Cuomo - who did not attend - down to the more than 1,200 union construction workers who collectively spent 3.2 million hours on the site.

"This is truly a project by and for New Yorkers," he said.

A view of the lobby of 4 World Trade Center, the first office tower to be completed on the original 16-acre World Trade Center site. (Susan Watts/New York Daily News)

Adding to the emotion of the day, two Silverstein Properties employees who were deeply involved in the construction and who lost loved ones on 9/11 attended the ribbon-cutting.

"I love this city, so when you're trying to bring it back, that's rewarding to be part of," said communications manager Jessica Schoenholtz-Murphy, whose brother Alex Steinman, a trader at Cantor Fitzgerald, died in terror attack.

Visitors were allowed to walk through a passage to the elevators at 4 World Trade Center. (STAN HONDA/AFP/Getty Images)

About a quarter of the space is leased to the Port Authority, which will use it as its new headquarters. And the city has rented a large chunk of the building for the Human Resources Administration. Roughly half of the building has not yet been rented out.

Japanese architect Fumihiko Maki, who designed the 72-story 4 World Trade Center, speaks at the opening ceremony Wednesday. (Susan Watts/New York Daily News)

With Wednesday's ceremony, those renting space can begin building offices.

The grand opening comes a day after the Council on Tall Buildings and Urban Habitat declared that its neighbor, One World Trade Center, will be the tallest skyscraper in the country when complete.

Nearby 1 World Trade Center can be seen from 4 World Trade Center. 1 World Trade Center is set to open for business in 2014. (Susan Watts/New York Daily News)

One World Trade Center, which is expected to open in 2014, beat out the Willis Tower in Chicago for the title.

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"We didn't have to be told that New York was number one, but it's always nice to hear," said Janno Lieber, president of World Trade Center Properties.

The Manhattan skyline as seen from the 68th floor of 4 World Trade Center in New York. (Susan Watts/New York Daily News)